1. Field of the Invention
This invention relates generally to broadband impedance matching transformers. More particularly, the invention relates to high impedance broadband transmission line transformers which exhibit low insertion loss, minimum voltage standing-wave ratios and provide a tuning network between balanced and unbalanced circuits.
2. Description of the Related Art
Transmission lines are used to connect various radio frequency circuit elements including connections from radio frequency (RF) circuits to antenna systems. Typical RF engineering practice dictates that a signal source should have an impedance equal to the impedance of the load. In addition, a load coupled to a transmission line should present an impedance equal to the characteristic impedance of the transmission line.
The importance of a matched load is that a transmission line terminated with a load equal to its characteristic impedance will transfer a signal without reflection. In that instance, all power contained in the signal is transferred from the transmission line to the load. Loads with a resistance unequal to the characteristic transmission line impedance produce reflections.
Short sections of transmission lines can be used to tune a mismatched load by inserting the section across the conductors as a shunt, or in series with the mismatched line. The length of the transmission line, the type of termination, (open or shorted), and its location determine the effect on the circuit. At very short wavelengths, transmission lines function as circuit tuning elements.
One application of a matching network would be employed at the output of an RF signal amplifier. A typical push-pull RF amplifier output stage would require an output transformer with a center tap for carrying equal, direct currents through each half of the primary winding to the transistors. The secondary winding provides a balanced output at a different impedance for conversion to an unbalanced line and for further circuit connection. A matched load is therefore essential to maximize power transfer.
A balun (BALanced-UNbalanced) is a passive device which permits a transition between an unbalanced circuit and a balanced circuit and also permits impedance matching if necessary. The balun provides electrical isolation, but passes the transmission line currents. Baluns avoid the high frequency limitations of conventional magnetic transformers since the windings are arranged such that winding capacitance and inductance form a transmission line free of resonances. Baluns can also provide impedance transformations with excellent broadband performance.
A prior art network converting a balanced output to an unbalanced output including an intermediate filtering network is disclosed in U.S. Pat. No. 5,495,212. However, the intermediate filtering network revealed does not perform a tuning function for the equivalent circuit; the network provides low-pass filtering.
While the prior art has shown impedance matching transmission line transformers using a combination of external devices incorporating intermediate filtering, the conventional devices are overly complex when designed to operate over a wide RF bandwidth. What is needed is a balanced-to-unbalanced transmission line transformer that permits tuning of the overall frequency response characteristics of the circuit.